Various types of bone anchors are used to engage implants and other devices to bone. In the spinal field, bone anchors are commonly used to attach rods, plates, and other types of stabilization devices and implants to one or more vertebrae. Such bone anchors include, for example, screws, bolts, pins, nails, hooks, clamps, staples and other types of anchor devices configured to be anchored in or to bone.
Bone anchors are typically formed of a single, biocompatible material. As a result, various portions of the bone anchor exhibit similar performance characteristics. For example, bone anchors may be formed of rigid materials, such as titanium or stainless steel. While the use of such rigid materials provides sufficient strength and load carrying capabilities to avoid fractures or breakage, the interface between the bone anchor and the surrounding bone is non-flexible and unyielding, which tends to decrease bone growth around the bone anchor. Alternatively, bone anchors may be formed of semi-rigid materials, such as polymeric materials. While the use of such semi-rigid materials provides a flexible or yielding interface between the bone anchor and the surrounding bone, which tends to increase bone growth around the bone anchor, the strength and load carrying capabilities of the bone anchor are reduced, thereby increasing the risk of fracture or breakage.
Additionally, in some applications, it may be desirable to provide one or more portions of the bone anchor with different performance characteristics, such as, for example, varying strength, rigidity, porosity, absorption rate, degradation rate, etc. To achieve this result, bone anchors may include multiple pieces or components formed of different materials, with the pieces or components assembled together to form the bone anchor. However, bone anchors that are assembled from multiple pieces or components tend to result in increased fabrication and assembly costs. Additionally, the components of an assembled bone anchor are subject to loosening and/or separation.
Thus, there remains a need for improved bone anchors having portions that exhibit different performance characteristics and methods of forming the same. The present invention satisfies this need and provides other benefits and advantages in a novel and unobvious manner.